@RandoMesher 

The right place to put second Deco unit, when it is not wired with Ethernet cable, is in the middle between your bedroom where you are and living room where your main Deco is.

Not in the bedroom. 

The main idea is: Main Deco to Second Deco will have better signal, and Second Deco to bedroom will have better signal. Overall - better signal.

Here, in more detail:

When looking for a place to put second Deco device, make sure 5 GHz signal from the main unit is strong (Good, Excellent) there. I used WiFi scanner on smartphone to check WiFi signal levels, WiFiman from Ubiquiti, but there are many other similar apps.

Walk to the place where you want to put second Deco M9 Plus, turn off and on WiFi on smartphone to get fresh signal readings, start WiFi scanner app and see what quality of 5GHz it reports from Main Deco. If quality is low, you better find another place.

Deco M9 Plus has 5GHz backhaul and unless wired with ethernet cable, will benefit greatly from strong 5GHz connection to Main Deco unit.

If you can't find such place, at least place M9 Plus where 2.4GHz signal from Main Deco is strong, but then you'll be losing main benefit of M9 and could have settled for (usually) less expensive M5.

Depending on your house layout, you might need to experiment with moving second Deco around to find sweet spot where it has good signal from main Deco and your bedroom gets good signal from second Deco.